Refactor.io - Share your code instantly for refactoring and code review
CodeTriage - Help out your favorite open source projects and become a better developer while doing it.
Codacy - Automatically reviews code style, security, duplication, complexity, and coverage on every change while tracking code quality throughout your sprints.
24 Pull Requests - 24 Pull Requests is a little project to promote open source collaboration during December.
CodeStream - CodeStream helps development teams resolve issues faster, and improve code quality by streamlining code reviews inside your IDE
Gitpay - Add bounties to solve Git issues from projects.